A majority of fluorescent lighting installations in current service use two F40T12 type rapid-start fluorescent lamps and a standard two-lamp, series-sequence, 40 watt rapid-start ballast to supply the necessary starting voltage and operating current. The F40T12 lamp is tubular in shape, being 48 inches in length and 11/2 inches in diameter. The F40T12 lamp and standard ballast combination is relatively inexpensive but is not as efficient in terms of lumens produced per watt of power consumed when compared to newer, more efficient fluorescent lamp and fixture designs. The efficiency of a conventional F40T12 lamp and standard ballast system may be improved by substituting higher efficiency T12-type lamps which use so-called "rare earth" phosphor coatings on the inside surface of the lamp. However, the higher efficiency T12-type lamps are significantly more expensive than the standard T12-type lamps which use a halo-phosphate type phosphor coating.
For a number of years, so-called "T8"-type fluorescent lamps have been available The T8-type lamp is similar in construction and materials to the conventional T12-type lamp except the T8-type lamp is one inch in diameter versus the 11/2 inch diameter of a T12-type lamp. T8-type lamps are available in a variety of lengths including a 48 inch length. Using a T8-type lamp in a light fixture is a better choice in a number of lighting applications. For example, a T8-type lamp using a rare earth phosphor coating, when installed in a light fixture, produces an equal light fixture output while consuming 25% less power in comparison to a T12-type lamp with a standard halo-phosphate phosphor coating. In another example, due to the lower materials cost resulting from its reduced size, a T8-type lamp using the standard halo-phosphate coating costs less than a T12-type lamp. In installations which are presently over lighted, a T8-type lamp using the standard halo-phosphate coating can be substituted for a T 12-type lamp, resulting in a 25% reduction in power consumption and a significant reduction in lamp cost, with only approximately an 8% reduction in light fixture output.
Consequently, it is desirable to be able to replace presently installed T12-type lamps with T8-type lamps to obtain their attendant advantages. Since T8-type lamps have the same connector pin configurations, spacing, and dimensions as T12-type lamps, physically substituting T8-type lamps for T12-type lamps is straightforward. The standard F40T12 lamp and standard ballast can be removed from the light fixture and replaced with a new ballast designed to operate T8-type lamps properly. However, this solution is prohibitively expensive because it essentially requires discarding the installed ballasts and purchasing new ballasts.
A better solution is to simply remove the F40T12 fluorescent lamps from the fixture and replace them with T8-type lamps having the same length as the F40T12 lamps. However, T8-type lamps and T12-type lamps are not electrically compatible. The standard two-lamp, series-sequence, 40 watt rapid-start ballast used to operate the F40T12 lamps is not compatible with T8-type lamps for two reasons. First, the starting voltage delivered by the ballast is too low to start a T8-type lamp. Second, the ballast impedance of the standard 40 watt T12-type ballast is too low to operate a T8-type lamp in a stable manner. A typical Argon-filled T8-type lamp requires an open circuit (i.e., starting) voltage of 315 volts and a peak voltage of 290 volts in order to start the lamp. However, the standard 40 watt T12-type ballast is only capable of delivering an open circuit voltage of 256 volts and a peak voltage of 240 volts. Consequently, the standard 40 watt T12 -type ballast cannot operate a T8-type lamp.
Therefore, an object of the present invention is to provide an adaptor circuit for allowing selected fluorescent lamps to operate using a predetermined ballast such that a user may choose a particular fluorescent lamp depending on desired illumination level and lamp cost.
Another object of the present invention is to provide a frequency selective adaptor circuit for use with a standard T12-type ballast which allows reliable, flicker-free operation of T8-type fluorescent lamps.
Another object of the present invention is to provide a frequency selective adaptor circuit for rapid-start type fluorescent lamps which allows a selected fluorescent lamp to reliably operate in a light fixture having a predetermined, installed ballast.
Yet another object of the present invention is to provide a cost effective alternative to ballast replacement which allows T8-type fluorescent lamps to efficiently and stably operate in a light fixture having a standard T12-type ballast installed therein.